Cylindrical fiber probes and methods of making them

ABSTRACT

This invention involves a fiber probe device and a method of making it. The probe includes a relatively thick upper cylindrical portion, typically in the form of a solid right circular cylinder, terminating in a tapered portion that terminates in a relatively thin lower cylindrical portion, typically also in the form of a solid right circular cylinder, the lower portion having a width (diameter) in the approximate range of as little as approximately 0.05 μm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of application Ser. No. 08/247165 filed May 20, 1994,which is a Continuation-In-Part of Ser. No. 08/091808 filed Jul. 15,1993, now abandoned.

FIELD OF THE INVENTION

This invention relates to probe devices, and more particularly tometrological fiber probe devices and to methods of making them.

BACKGROUND OF THE INVENTION

More than 100 years ago, the famous physicist Ernst Abbe described afundamental limitation of any microscope that relies on any lens orsystem of lenses in an imaging system to focus light or other radiation:diffraction obscures (makes fuzzy) those details of the image that aresmaller in size than approximately one-half the wavelength of theradiation. See "Scanned-Probe Microscopes" by H. Kumar Wickramasinghe,published in Scientific American, Vol. 261, No. 4, pp. 98-105 (October1989). In other words, the resolution of the microscope is limited bythe wavelength of the radiation. In order to circumvent this limitation,researchers have investigated the use of inter alia, involving"near-field scanning optical microscopy" (hereinafter "NSOM") devices.These devices typically comprise an aperture located at the tip of anelongated optical probe, the aperture having a (largest) dimension thatis smaller than approximately the wavelength of the optical radiationbeing used, positioned in close proximity to the surface of a samplebody; and the aperture is allowed to scan (move) laterally (in one ortwo dimensions) across the (irregular) surface of the sample body atdistances of separation therefrom all of which distances arecharacterized by mutually equal force components exerted on the probedevice in the direction perpendicular to the global (overall) surface ofthe sample body, the scanning being detected and controlled by anelectromechanical feedback servomechanism.

For example, U.S. Pat. No. 4,604,620 inter alia describes a probe devicehaving an aperture located at the tip of a cladded glass fiber that hasbeen coated with a metallic layer. The aperture is drilled into themetallic layer at the tip of the fiber at a location that is coaxed withthe fiber. The (immediate) neighborhood of the tip is composed of asection of solid glass fiber that has obliquely sloping (truncatedconical) sidewalls, whereby the sidewalls do not form a cylinder of anykind. Therefore, as the probe device laterally scans a rough surface,the calculations required to determine the desired information on theactual contours (actual profile) of the surface of the sample bodyrequire prior detailed knowledge of the slanting contours of thesidewalls of the probe, and these calculations typically do not yieldaccurate metrological determinations of the desired profile of thecontours of the surface of the sample body, especially at locations ofthe surface of the sample body where sudden jumps (vertical steps)thereof are located. In addition, fabrication of the probe device iscomplex and expensive, especially because of the need for drilling theaperture coaxially with the fiber.

SUMMARY OF THE INVENTION

This invention involves, in a specific device embodiment, a probe devicecomprising a fiber having a relatively thick upper cylindrical portionterminating in a tapered portion that terminates in a relatively thinright cylindrical lower portion; the lower cylindrical portion havingmaximum width in the approximate range of 0.05 μm to 10. μm andterminating at its bottom extremity in an essentially planar end surfaceoriented perpendicular to the axis of the thin right cylindricalportion. Other embodiments are specific in the dependent device claims.As used herein, the term "maximum width" refers to the maximumdiameter--i.e., the length of the longest line segment that can be drawnin a cross section of the fiber, oriented perpendicular to the axis ofthe cylinder, from one extremity of the cross section to another.

The invention also involves a method of making such a probe device, asspecified in the method claims and in greater detail in the DetailedDescription below.

The fact that the lower portion of the probe device terminates in aplanar end surface--advantageously oriented perpendicular to the axis ofthe cylinder--enables accurate positioning and henceposition-determinations of the probe at locations of a surface of asample body being scanned by the probe, even at sudden jumps in thesurface. And the fact that the lower portion of the probe device has theform of a cylinder simplifies the determination of the profile of thesurface of the sample body.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1-4 depict cross sections of a probe device being fabricated inaccordance with a specific embodiment of the invention. Only for thesake of clarity, none of the FIGURES is drawn to any scale.

DETAILED DESCRIPTION

Referring to FIG. 1, a glass fiber segment 10 takes the form of a solidfight circular cylinder. A top portion of the sidewall surface of thissegment 10 is coated with a polymer resist layer 20 that is resistant tohydrofluoric acid etching. The glass fiber segment 10 has a bottom endface 11 that is flat and is oriented in a plane perpendicular to theaxis of the (cylindrical) segment 10. A top face of the segment 10 iscoated with a bonding layer 30, such as a layer of epoxy or othercement, whereby the segment 10 is bonded to a holder 40, typically madeof teflon. Instead of a cement layer, a thin layer of suitable materialcoated with an adhesive layer on its top and bottom surfaces can beused.

Advantageously, the polymer resist layer 20 is a chlorofluorocarbonpolymer dissolved in an organic solvent typically comprising a ketone oran ester or a mixture of a ketone and an ester. For example, the polymerresist is a copolymer formed by polymerizing vinylidene fluoride andchlorotrifluoroethylene commercially available as a resin from 3MCorporation under the tradename "KEL-F" Brand 800 resin, which isdissolved in amyl acetate or other suitable organic solvent to theextent of approximately 30-to-50 wt percent resin.

The fiber segment 10 is immersed (FIGS. 1 and 2) in a wet isotropicetch, typically a buffered oxide etching solution 50--such as a solutioncomposed of 2 parts buffered (7:1) oxide etch, 1 part hydrofluoric acid,and 1 part acetic acid. The etching solution 50 is contained in acontainer 60, and it has a level 51 that intersects the resist layer 20somewhere, whereby the entire (lower) portion of the surface of thefiber segment 10 that is not coated with the resist layer 20 issubmerged in the solution 50. After the fiber segment 10 has thus beenimmersed for a prescribed amount of time, it assumes the shape shown inFIG. 2--that is, relatively a thick upper portion 23, in the form of asolid right circular cylinder, terminating in an undercut intermediatecylindrical portion 22, in the form of a tapered truncated circularpyramid, terminating in a relatively thin lower cylindrical portion 21,in the form of another solid fight circular cylinder.

For example, the height (length) H (FIG. 1) of the bottom portion of thefiber segment 10, which is not coated with the resist layer 20, istypically equal to approximately 2.5 cm; and the diameter D (FIG. 1) ofthe fiber segment 10 is typically equal to approximately 125. μm ormore. After having been etched with the solution 50, the thin lowerportion 21 has a diameter 2R (FIG. 2) equal to approximately 50. μm, asdetermined by the duration of the immersion.

Next, the bottom face of this lower portion 21 is then cleaved in aplane oriented perpendicular to the (common) axes of the upper portion23 and the lower portion 21, as by means of fiber cleaver aided opticalmicroscopic viewing or other micrometer controlled procedure. In thisway, the height of the lower cylindrical portion 31 is reduced to apredetermined value h (FIG. 3), and the tip thereof is a planar surfaceoriented perpendicular to the axis of this lower cylindrical portion 31.Typically, this height h is equal to approximately 5. μm. The resistlayer 20 (FIG. 1) is then removed ("stripped"), or it can be removedprior to the cleaving, such as by immersion either of the entire or ofonly a bottom portion of the resist layer 20 in acetone: whereby eithernone or only a top portion (not shown) of the resist layer 20 remains.

The fiber segment again is immersed (FIG. 4) in the etching solution 50,for another prescribed time duration, to a solution level 52 thatintersects the segment at a level located at its thick upper portion andthat isotropically etches the fiber. In this way, the resulting lowerportion 41 of the fiber segment is still a solid fight circular cylinderbut having a reduced diameter equal to w, while the height h thereof isreduced by an unimportant amount. Likewise, the diameters of theresulting intermediate portion 42 and the upper portion 43 of the fiberare reduced by unimportant amounts. At the location of the level 52, ameniscus of the etching solution 50 produces an unimportant gradualtransition between regions of the fiber immediately above andimmediately below the solution level 52, as indicated in FIG. 4.

The lower portion 41, intermediate portion 42, and upper portion 43 alltake the form of mutually coaxial solid circular cylinders. The diameterw of the lower portion 41--i.e., the width of the tip of the resultingprobe (FIG. 4)--can be adjusted to any desired value by adjusting theamount of time during which the immersion in the solution 50 is allowedto continue. This width w can be as small as approximately 0.05 μm andas large as approximately as 10. μm or more--typically in theapproximate range of 0.05 μm to 0.2 μm--depending on the ultimatelydesired metrological use of the probe, i.e., depending on the desiredmetrological resolution of the probe during subsequent use as a probedevice. Typically, such use involves scanning the surface of a samplebody with the probe while holding the probe with a electromechanicalfeedback servo-mechanism, as known in the art, that maintains the tip ofthe probe at distances of separation from the surface, all of whichdistances are characterized by mutually equal components of force in thedirection perpendicular to the overall surface of the sample body. Theprescribed time durations of the immersions for the etchings (FIGS. 1-2and 4) can be determined by trial and error.

Although the invention has been disclosed in detail in terms of aspecific embodiment, various modifications can be made without departingfrom the scope of the invention. For example, instead of glass, thefiber 10 can be made of any material--such as silica--that can beselectively etched by means of a solution while the fiber is selectivelyprotected with a resist layer, and that can be cleaved to form a(planar) tip. Instead of isotropic wet etching, other kinds of etchingtechniques can be used, such as dry plasma etching. The etchings areadvantageously, but need not be, isotropic. The two etching solutions(FIGS. 1-2 and 4) can be chemically different or physically different(e.g., wet in FIGS. 1-2, dry in FIG. 4).

We claim:
 1. A method of making a probe device comprising the stepsof:(a) providing an initial cylindrical fiber; (b) coating an uppercylindrical portion of the fiber with a protective layer that protectsthe upper cylindrical portion from etching during step (c) but does notprotect a lower cylindrical portion, said lower cylindrical portionterminating in a tip; (c) etching, prior to step (d), the lowercylindrical portion of the fiber for a first controlled time duration,whereby the width of the lower cylindrical portion is reduced, but thewidth of the upper cylindrical portion remains essentially unchanged,(d) cleaving the tip of the lower portion of the fiber, whereby acleaved lower portion of the fiber is formed; and (e) etching,subsequent to step (d), the cleaved lower portion of the fiber for asecond controlled time duration, whereby the width of the cleaved lowerportion is further reduced.
 2. The method of claim 1 in which theetching of step (c) is essentially an isotropic etching step.
 3. Themethod of claim 2 in which the etching of step (c) is wet etching. 4.The method of claim 2 in which the etching of step (e) is essentiallyisotropic etching step.
 5. The method of claim 4 in which the etching ofstep (e) is wet etching.
 6. The method of claim 1 in which the etchingof step (e) is essentially isotropic.
 7. The method of claim 6 in whichthe etching of step (e) is wet etching.
 8. The method of claim 5 inwhich the width of the cleaved lower portion is reduced to lie in theapproximate range of 0.05 μm to 10 μm.
 9. The method of claim 5 in whichthe width of the cleaved lower portion is reduced to lie in in theapproximate range of 0.05 μm to 0.2 μm.
 10. The method of claim 1 inwhich the initial cylindrical fiber is essentially glass.
 11. The methodof claim 2 in which the initial cylindrical fiber is essentially glass.12. The method of claim 1 in which the protective layer is formed bydissolving a chlorofluorocarbon polymer resin in an organic solvent. 13.The method of claim 12 in which the initial cylindrical fiber isessentially glass.
 14. The method of claim 1 in which the protectivelayer is formed by dissolving a chlorofluorocarbon polymer resin in aketone or an ester or a mixture of a ketone and an ester.
 15. The methodof claim 14 in which the initial cylindrical fiber is essentially glass.16. The method of claim 2 in which the protective layer is formed bydissolving a chlorofluorocarbon polymer resin in an organic solvent. 17.The method of claim 16 in which the initial cylindrical fiber isessentially glass.
 18. The method of claim 2 in which the protectivelayer is formed by dissolving a chlorofluorocarbon polymer resin in aketone or an ester or a mixture of a ketone and an ester.
 19. The methodof claim 18 in which the initial cylindrical fiber is essentially glass.20. The method of claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,15, 16, 17, 18, or 19 in which the cleaving of step (d) is orientedperpendicular to the axis of the lower cylindrical portion.
 21. A methodin accordance with claim 20 followed by moving the probe device atdistances from a surface of a sample body where the forces exerted bythe sample body on the probe device are mutually equal.
 22. A methodincluding making the probe device in accordance with claim 1, 2, 3, 4,5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, or 19 followed bymoving the probe device at distances from a surface of a sample bodywhere the forces exerted by the sample body on the probe device aremutually equal.